An overlay analysis is the process of overlaying 2 or more spatial layers and capturing statistics associated with their relative coverage. In this case, the sub-watershed layer is overlain by Provincial land-use and surficial geology layers to obtain information like percent impervious, relative permeability, etc.
Provincial layers discussed in more detail below have in all cases been re-sampled to the 50x50m² grid associated with the hydrologically corrected DEM. It is from these rasters where the aggregation of watershed characteristics is computed.
The Ministry of Natural Resources and Forestry (2019) SOLRIS version 3.0 provincial land use layer is employed to aggregate imperviousness and canopy coverage at the sub-watershed scale. In areas to the north, where the SOLRIS coverage discontinues, interpretation was applied by:
The dominant SOLRIS land use class (by area) is assigned the Land use class index for every 50x50m² grid cell.
Final 50x50m SOLRIS mapping. (For illustrative purposes only see here to reproduce shown raster.)
For any ~10km² sub-watershed and give a 50x50m² grid , there should be a set of roughly 4,000 SOLRIS land use class indices. Using a look-up system, the set of cells contained within a sub-watershed are assigned a value of imperviousness, water body, wetland and canopy coverage (according to their SOLRIS index) and accumulated to a sub-watershed sum.
Percent impervious and canopy coverage as per SOLRIS v3.0 (MNRF, 2019) land use classification.| Index | Name | Imperviousness (%) | Canopy cover (%) |
|---|---|---|---|
| 11 | Open Beach/Bar | ||
| 21 | Open Sand Dune | ||
| 23 | Treed Sand Dune | 50 | |
| 41 | Open Cliff and Talus | ||
| 43 | Treed Cliff and Talus | 50 | |
| 51 | Open Alvar | ||
| 52 | Shrub Alvar | 25 | |
| 53 | Treed Alvar | 50 | |
| 64 | Open Bedrock | ||
| 65 | Sparse Treed | 40 | |
| 81 | Open Tallgrass Prairie | 10 | |
| 82 | Tallgrass Savannah | 35 | |
| 83 | Tallgrass Woodland | 85 | |
| 90 | Forest | 100 | |
| 91 | Coniferous Forest | 100 | |
| 92 | Mixed Forest | 100 | |
| 93 | Deciduous Forest | 100 | |
| 131 | Treed Swamp | 50 | |
| 135 | Thicket Swamp | 50 | |
| 140 | Fen | 25 | |
| 150 | Bog | 25 | |
| 160 | Marsh | 25 | |
| 170 | Open Water | ||
| 191 | Plantations – Tree Cultivated | 85 | |
| 192 | Hedge Rows | 85 | |
| 193 | Tilled | ||
| 201 | Transportation | 85 | |
| 202 | Built-Up Area– Pervious | 10 | 10 |
| 203 | Built-Up Area– Impervious | 90 | |
| 204 | Extraction–Aggregate | ||
| 205 | Extraction –Peat/Topsoil | 10 | |
| 250 | Undifferentiated |
(For illustrative purposes only see here to reproduce shown raster.)